1. Field of the Invention
This invention relates to an optical data link, especially a light-receiving circuit used in the optical data link.
2. Related Prior Art
A conventional optical data link that uses a monitoring circuit of photo current generated by an optical device is disclosed in the U.S. Pat. No. 6,188,059. The disclosed monitoring circuit detects a current proportional to the photo current flowing in the optical device, such as a photo diode, by a current-mirror circuit.
On the other hand, a signal detection circuit may be provided in the conventional optical data link. The signal detection circuit detects through the photo current whether an optical signal is inputted into the photo diode or not. This signal detection circuit may be provided in one current path different to a path for monitoring current of the current-mirror circuit.
However, in the configuration above mentioned where the signal detection circuit and the current monitoring circuit are provided in respective path of the current-mirror circuit, some inconvenience described below may occur. When an ammeter is connected to one terminal for current monitoring of the current-mirror circuit, some bias voltage will be applied to the terminal. On the other hand, when any current monitoring is not necessary and accordingly any circuit is not connected to the terminal for current monitoring, no bias voltage will be applied, which affects conditions of the signal detection. Thus, the signal detection circuit may depend on conditions of the paired path of the current-mirror circuit. When the photo diode is an avalanche photo diode (herein after denoted by APD), the magnitude of the current flowing the current-mirror circuit must be as small as possible to save the power consumption of the power source that supplies the bias voltage to the APD. Accordingly, the conditions of the current path of the current-mirror circuit remarkably affect the signal detection.